The present invention relates to a method for monitoring a moving web, such as a wire in a paper machine, a felt or a material web, such as a board or paper web, in which a radiation bar is directed at the face of the web to be monitored by a series of transmitters. The radiation bar is arranged transverse to the direction of movement of the web, and radiation reflected from the web and the background is detected by means of a series of receivers. Measurement signals obtained from the series of receivers is passed to an electronic unit and a quantity that characterizes the monitoring of the web is detected from the measurement signals.
The present invention also relates to a device for monitoring an edge or equivalent of the moving web, comprising a measurement head, which is arranged free of contact with the web and in proximity to the web to be monitored. The measurement head includes a series of radiation transmitters and a series of radiation receivers, which receive the radiation derived from the radiation transmitters and reflected from the web to be monitored and from the background. On the basis of the reflected radiation, it is possible to form a measurement signal or a series of measurement signals which represent the position of the edge or edges of the web and/or the distance between the edges and/or a point of discontinuity in the web.
In paper machines and other paper forming equipment, in which a continuous material web is manufactured and/or closed fabric loops are employed, it is necessary to identify the location and/or the presence of the edge of the material web or the fabric in various stages of the paper-making process. For this purpose, photoelectric means of identification are typically used. These prior art identification devices usually operate so that a source of light and a photocell are placed at opposite sides of the fabric or web to be monitored. A break of the web and/or a shifting of the edge of the web or fabric results in the photocell receiving the beam of light, transmitting an impulse further, which results in an alarm and possibly in other responsive action.
In the monitoring of an edge of a wire in a paper machine, it is known in the prior art to use three photocells placed side by side, wherein each of the cells operates by an "on/off" principle. The precision of this monitoring device and/or the width of the area of monitoring has/have, however, proved unsatisfactory in several applications.
Also, in the prior art, various identification devices are known which are based on reflection of light taking place from the material to be monitored and on changes occurring in the reflection. As an example of such devices, reference is made to U.S. Pat. No. 4,146,797, wherein an identification device for locating the edge of a material web is described. The device includes a source of light and a detector of light. The source of light in this device directs a spot of light at the lateral area of the web to be monitored. The position of the lateral area is monitored and the position of the edge of the web is measured on the basis of changes taking place in the intensity of the reflected light.
Prior art photoelectric identification devices do not operate adequately in all respects, but disturbances occurs in them, and they require constant supervision, frequent calibration and cleaning. Paper machines are also an operational environment that impose very high requirements because of high temperature, moisture, and impurities, which produce disturbance in the prior art photoelectric identification means. With the constantly increasing running speeds of paper machines in existence and as planned in the near future, these problems have increased further.
The increasing running speeds of paper machines are also influential in this connection whereby ever higher and more demanding requirements are imposed on the regulation systems that control the transverse positions of the fabrics in the paper machine. These requirements are applied in particular to the precision and rapidity of the measurement of the position of a fabric.
The operation of the prior art web-monitoring devices is also disturbed by the background light and by the changes in same, such as oscillations in fluorescent tubes arising from the mains frequency. In the environment of a paper machine, there is also a considerable amount of infra-red radiation. Since most photocells are sensitive to infra-red radiation, this causes disturbances in the photoelectric identification means.
With respect to prior art in the same field as the present invention, reference is made to U.S. Pat. No. 4,146,797 and to the assignee's Finnish Patent Application No. 910571 (corresponding to EP Appl. No. 91 850153.7 and U.S. Ser. No. 07/719,762, the specification of which is hereby incorporated by reference herein). In FI 910571, a method and a device are described for identifying a moving material web, wherein a beam of light is directed at the material web by means of a transmitter device. The beam of light causes a second beam of light to be reflected from the face of the material web to be identified. The second, reflected beam of light is converted by a receiver device into an electric signal. On the basis of the electrical signal, the presence, quality, condition, and/or position of the material web is/are identified. The intensity of the beam of light transmitted from the transmitter of light is regulated on the basis of the intensity of the reflected beam of light. The reference level(s) of the electric identification signal derived from the reflected beam of light is adapted in compliance with the environment of operation so as to optimize the identification and to minimize interference from the environment. The device also includes a microprocessor, to which an analog signal is passed through an A/D converter. The microprocessor controls the unit for regulation of the intensity of the light to be transmitted, which unit controls an adjustable voltage source from which a regulated operation voltage is supplied to the transmitter of light.